Yong-Ahn Park

Morphology, sedimentology and tidal channel processes on a high-tide-range mudflat, west coast of South Korea

Abstract Most of the west coast of Korea is fronted by broad intertidal sand and mudflats which have formed in a high-tide-range environment that is subjected periodically to intense winter-season surges from monsoonal winds. The lack of a large source of sediments and the absence of a protective barrier at their seaward margins makes these tidal flats unusual, if not unique. Field studies, conducted in 60 km2 of an embayed region just south of Inchon, were undertaken for the dual purpose of describing tidal channels and morphology of surrounding tidal flat surfaces, and assessing the role of tidal channels in processes of sedimentation. This paper discusses morphologic variability, channel geometry, and processes of intertidal sedimentation; the following companion paper addresses internal hydraulics of channel flow and the effects of observed internal motions on the erosion, transport, and deposition of sediments. Results show a highly variable morphology with locally dense drainage channel networks. North-south transects revealed that the intertidal surface was actually terraced by step-like changes in elevation at three of the channels, producing 2–3 m of intertidal relief over a distance of only a few kilometers. Whereas the northern, terraced tidal flat was characterized by tightly meandering tributary channels and sediments that were 80–90% mud, the southern region contained coarser sediments and lacked tributary channels altogether. Most channels appeared to be ebb dominated; peak speeds of 60–70 cm/s that were vertically homogeneous throughout the core of the water column were capable of producing intense shears near the bed and suspended-sediment concentrations on the order of 3000 mg/l. During ebbing tide, transmissometer profiles showed step structures, reverse gradients, and variability in the frequency range 0.03–0.008 Hz (∼ 30 s–2 min). Although most of the energy was concentrated in spectral peaks at 125, 95, 66, and 50 s, approximately 40% of the records contained variance at frequencies too low to be resolved. Surges in transparency and associated sediment billows 2–3 h before low tide were a product of internal flow instabilities. A record of the first summer (monsoonal) storm showed enhanced ebb flows and transparency values near zero between peak ebb and peak flood. A 5–10 cm thick layer of soft, fluid-like mud was eroded from the tidal flat surface during the storm and probably explains the low transparency values. The combined erosion by storms and daily dispersal by channelized ebb currents requires a substantial return of sediments in order for the tidal flats to maintain their elevation. The apparent lack of bathymetric relief in the subtidal region indicates that either sediments carried seaward in ebb channels are removed by shelf processes (and thus returned at some later time) or are simply reintroduced to the intertidal region by flood currents.

Late Pleistocene silty tidal rhythmites in the macrotidal flat between Youngjong and Yongyou Islands, west coast of Korea

Abstract Late Pleistocene silty tidal rhythmites, developed in the macrotidal flat between Youngjong and Yongyou Islands, demonstrate hierarchical tidal cycles ranging from daily to monthly variations associated with synodic, tropical and anomalistic cycles. On the basis of rhythmicities and textural composition, the rhythmites were likely deposited in an upper intertidal flat under a mixed, dominantly semi-diurnal, tidal regime. The rhythmites, showing gradational contact with overlying cryoturbated silty facies indicating subaerial exposure and subsequent weathering during the Last Glacial Maximum, represent the tidal environment during the last highstand or ensuing regression phase of sea-level in the late Pleistocene.

Late Quaternary stratigraphy and evolution of a Korean tidal flat, Haenam Bay, Southeastern Yellow Sea, Korea

Abstract Sediment cores from a tidal flat in the southeastern Yellow Sea, Korea, contain two major depositional units, a Holocene unit (Unit I) and an underlying late Pleistocene unit (Unit II). Unit I (up to 15 m thick) is a middle to late Holocene, coarsening-upward transgressive deposit. Unit II (about 10 m thick) is interpreted as tidal-flat deposits from the last interglacial period (probably oxygen-isotope stage 5e), based on its many tide-influenced signatures such as laminated silt/clay, tidal rhythmites, fossil crab burrows, authigenic minerals (pyrite and halite), clay mineral composition and a dinoflagellate cyst assemblage comparable to that of Unit I. Units I and II are separated by a regional unconformity which is highlighted by a yellow ‘oxidized layer’ in uppermost Unit II. The layer contains abundant evidence of subaerial exposure, including a yellowish sediment color, semi-consolidation, desiccation cracks, cryoturbation structures, and high degree of magnetic susceptibility. Two major interglacial highstands (Units I and II) and one glacial lowstand (uppermost oxidized layer of Unit II) thus characterize coastal to nearshore tidal deposits along the eastern margin of the Yellow Sea (western Korea).

Internal hydraulics of a sediment-stratified channel flow

Abstract Mudflats along the macrotidal Yellow Sea coast of South Korea are incised by numerous channels that act as drainage and sediment dispersal routes, primarily during ebb flows. Observations in two channels at maximum ebb revealed stable vertical distributions of suspended sediment (density) capped by a lutocline at mid-depth, across which suspended-sediment-related density varied rapidly. Internal responses, as revealed by turbidity and velocity data, varied with flow, suspended sediment, and morphology. At one location, upstream-propagating lowest mode internal waves with period T = 140 s were observed. Superimposed on the interfacial waves were higher frequency Kelvin-Helmholtz billows, apparently related to mean flow shear. Visual observations indicated that billows filled the channel from surface to bottom. At the second location, an undulatory jump consisting of a series of surges was observed. Each surge resembled the head of a turbidity current. Kelvin-Helmholtz billows were present, but only at surge crests and troughs. Internal hydraulic jumps, sediment spilling from feeder channels, and collapse of instabilities may contribute to wave generation. A range of bottom sediment sizes permits the development of a stable stratification within the bottom boundary layer beneath a lutocline. A simple numerical model shows that the lutocline is the upper limit of a boundary layer with a thickness proportional to mean flow velocity and to the intensity of suspended-sediment stratification within the layer. Billows appear to enhance resuspension of sediment, thereby increasing the sediment load and thickness of the boundary layer. Locations of hydraulic jumps appear to be sites of deposition.

Sedimentology of a subtidal, tide-dominated sand body in the Yellow Sea, southwest Korea

Abstract Odanam Satoe, a subtidal, tide-dominated sand body in the Yellow Sea, Korea, is linear in plan and asymmetrical in cross-section. It consists of fine- to medium-grained, well-sorted subangular sand. Bedforms consist of high-amplitude (1–2 m) sandwaves on the lower flanks of the gentler-sloping bar surface, and medium-amplitude (0.5-1 m) sandwaves on the sand body trough adjoining the steeper face, the bar crest and shallower parts of the gently sloping bar surface. Bedforms are absent on the relatively steeper bar surface, which is characterized by 2° slopes. Bedform orientation on the gentler slope is oblique by 30° to the bar crest, parallel to the sand-body crest on the crest itself, and opposite to the steeper sand-body face in the trough below the steeper slope of the bar. Bottom current velocity data show that tidal currents are semi-rotary with a flood time—velocity asymmetry over the gentler slope, and ebb time—velocity asymmetry over the steeper slope during most of the tidal cycle. Tidal-current flow parallels bar elongation over the steeper slope, whereas over the gentler slope, tidal-current flow is directed at 30° to the bar crest and changes to normal to the crest one hour prior to low tide. Bedform orientation mapped with side-scan sonar shows agreement with these flow directions. Sand dispersal around the sand body is controlled by time—velocity asymmetry and partial rotary flow directions of tidal currents. This circulation causes not only a trapezoidal mode of grain dispersal, but also westerly migration of the sand body documented from comparative bathymetric surveys in 1964 and 1980.

Observations on shelf and subtidal channel flow: Implications of sediment dispersal seaward of the Keum River estuary, Korea

Although much of the fine-grained sediment in the south-eastern Yellow Sea is attributed to discharge from the Keum River, dispersal of these sediments by tide or wind has never been determined by direct current measurements. In this paper we provide in situ observations of currents in two dispersal systems immediately seaward of the Keum River entrance. One of these is a subtidal channel system that defines flow from the shallow subtidal region seaward to the shelf, and the other is the inner shelf mudstream where mixing is maintained primarily by tides. Subtidal channels have moderate current speeds but low suspended sediment concentrations. Instantaneous current speeds during spring tide were of the order 100 cm s −1 and suspended sediment concentrations were typically less than 10 mg l −1 at the surface and 40 mg l −1 at the bottom. Under fair weather conditions, subtidal channels play a negligible role in offshore sediment dispersal since sediment concentrations during flood appear to equal or exceed those during ebb. Instantaneous currents at two stations on the shelf were of the same order as those in subtidal channels, but displayed net flow to the south at speeds of 7·8 and 3·8 cm s −1 . We speculate that subtidal channels provide the conduits for transporting sediments directly offshore during winter monsoons. On an annual time scale, one model for sedimentation would be low sediment influx from rivers but high influx from tidal flats to the coast during winter, when north winds generate large waves in the Yellow Sea, and the reverse in summer, when winds from the south produce little wave activity. Coastal muds would thus be eroded in winter and replenished by high river discharge in summer. Further, it is reasonable to assume that residual flow to the south would strengthen in winter as the monsoon season progressed. Thus the introduction of resuspended tidal flat sediments would coincide with the best developed dispersal system.

Occurrence of authigenic siderites in the Early Holocene coastal deposit in the west coast of Korea: an indicator of depositional environment

The macrotidal muddy deposits around Youngjong Island in the west coast of Korea with a radiocarbon dating of ca. 8000 yr BP contain micro-scale (ca. 150 μm) siderite concretions. The authigenic siderites exhibit the aggregated spherulitic textures with well-developed rhombs on the grain surfaces, which reflects good preservation. Geochemical analyses indicate that the concentrations of Mg (mean 2.6 mol%) and Ca (mean 9.4 mol%) are low whereas those of Fe (mean 70.4 mol%) and Mn (mean 17.6 mol%) are relatively high. The chemical composition suggests that the authigenic siderites might have precipitated in a non-marine environment. The occurrence of such siderites in the early Holocene sediments, considering the stratigraphic level with respect to the well-established sea-level curve helps recognize the non-marine depositional environment existed before the formation of modern tidal flats along the west coast of Korea.

Storm-induced fine-sediment transport, west coast of south Korea

Archived data, together with field observations collected between 1980 and 1982 off the west coast of Korea, suggest that the onset of winter monsoon winds in late fall initiates a residual southward flow that potentially carries large volumes of resuspended bottom sediments into the Korea Strait. During the calm conditions of summer, muds are replenished by high river discharges and reform the band of soft material, which characteristically occurs as a series of mudflats near the coast. Thus, these mudflats serve as a temporary storage facility during summer accumulation and as a source during winter erosion.

Elemental composition of siderite grains in early-Holocene deposits of Youngjong Island (west coast of Korea), and its palaeoenvironmental implications

Macrotidal muddy coastal deposits around Youngjong Island (west coast of Korea, eastern Yellow Sea) comprise four lithostratigraphic units (unit I, unit II, unit III and unit IV). Overlain unconformably by unit I of modern muddy intertidal sediments, Unit II of early-Holocene age (ca. 8000 yr BP) is characterized by a distinct color, peat fragments, rootlets, and especially microscale (ca. 150 μm) siderite grains whose mineralogy was confirmed by X-ray diffraction. Siderite concretions occur as aggregated spherulitic forms with texturally well-developed rhombs on the grain surface, as verified by means of scanning electron microscopy. Electron microprobe analyses indicate that the siderite grains have low contents of Mg (<3 mol%) and Ca (<8 mol%). In contrast, high contents of Fe (ca. 70 mol%) and Mn (ca. 15 mol%) suggest that the siderite precipitated in a non-marine depositional environment such as freshwater bogs. The occurrence of such diagenetic and authigenic siderites in early-Holocene sediments highlights a potential for providing information on depositional environments for which unequivocal sedimentological evidence is otherwise lacking, such being the case for the muddy coastal deposits in the eastern Yellow Sea.